Setting Voxel Coordinates
In the Blender wiki article it says


  The voxels are stored in a flat z/y/x grid of floats.


That means that the coordinates of every density is defined by its position in the file. I guess that your original data set is sorted that way too. For example:

xx yy zz dd
-----------
12 20 00 dd
12 20 01 dd
12 20 02 dd
...
12 21 00 dd
12 21 01 dd
...


In this case the coordinates are redundant because they can be easily deduced from the position in the list. Thus the coordinates can be omitted.

Also the file format doesn't use ASCII characters. It is a binary file and simply stores numbers. Every number can be interpreted as an ASCII character. The character for 64 is @. But in this case this is irrelevant.